Abstract

A sequence of sixteen photomicrographs of
thin sections of
unaltered quartz diorite through a zone of deformation to
myrmekite-bearing granite near Temecula, California, shows the textural
and mineralogical changes that occurred in a quartz diorite as (1)
K-metasomatism altered the primary plagioclase crystals to form
microcline, myrmekite, quartz-bleb clusters, and recrystallized sodic
plagioclase and as (2) Si-metasomatism converted microfractured biotite
and hornblende into quartz. Cathodoluminescence, electron microprobe, and
scanning electron studies confirm the chemical changes that occurred in
the altered and recrystallized minerals. These photomicrographs
and others (a) provide
clues to the origin of metasomatic granitic in
other terranes, (b) show that massive granite lacking gneissic fabric
or cataclasis can be formed by K-metasomatism, (c) indicate that sharp
contacts between mafic and felsic igneous rocks are not always caused
by injection of magma into fractured rock, (d) reveal that complete
cataclasis of all normally zoned plagioclase crystals in a mafic rock
followed by K-metasomatism results in
metasomatic granite lacking both normally zoned plagioclase and
K-feldspar megacrysts, and (e) suggest that selective deformation of a
few normally zoned plagioclase crystals in a primary rock can result
in K-feldspar megacrysts coexisting with normally zoned plagioclase in
granodiorite or quartz monzonite.

Introduction

This article is intended to be a supplement
to articles 1 and 2
(Collins, 1997a, 1997b and to provide additional illustrations
that
show how myrmekite formed in the site near Temecula, California (Collins,
1988ab; Hunt et al., 1992). These illustrations expand the evidence to
show how myrmekite can be used as a clue to metasomatic transformations on
a plutonic scale in other localities. The description of the field
location and outcrops in the Temecula site are given in Collins (1988b).
Fig. 1, Fig. 2, and Fig. 3 from this publication show the general locations
of the samples from which thin sections and photomicrographs were
obtained.

Geologic setting and generalized petrography

Southwest of Temecula the Bonsall tonalite
and the San Marcos
gabbro intrude the Julian schist, and then the Woodson Mountain
granodiorite intrudes all three of these rock types(Fig. 1,
Fig. 2, and Fig. 3). The
Bonsall tonalite is labeled diorite on Fig. 3,
but hereafter it is called quartz diorite because locally in the
area of study it contains quartz. Granitic dikes, extending from the
Woodson Mountain granodiorite and penetrating this quartz diorite (Fig. 3), are exposed in stream polished outcrops
along the Santa Margarita River. The
granitic dikes contain about 55-60% microcline, 30-35% quartz, 1-3%
biotite, 5-10% plagioclase An12-15, and up to 1% myrmekite (Fig. 4. Where the quartz diorite is unaltered (west of
photo 6, Fig.
3), it consists of 5-15% biotite, 5-15% hornblende, 60-75%
plagioclase, and 5-10% quartz. The plagioclase is normally zoned (cores
An39; rims An20 and albite twinned (Fig. 5). On the basis of electron-microprobe studies,
this plagioclase lacks any K in the cores or rims except in parts per
million or less. Hornblende and biotite lack quartz in their cores (Fig. 5) and have no apparent alteration. The quartz
diorite and the granite dikes (Fig. 3) are cut by an
amphibolite dike (a former andesite
porphyry; Fig. 6), and the quartz diorite also
contains xenoliths of older amphibolite. Compositions of both
amphibolites
are nearly the same but slightly more mafic than the quartz diorite (Fig. 5). Barely-visible narrow remnants of quartz
diorite (3-6 cm wide) border the amphibolite dike on both sides where the
amphibolite dike extends through the granite, and narrow fingers of the
amphibolite dike extend into remnant quartz diorite but not into the
granite (Fig. 7).

The amphibolite dike has the first
appearance of being younger
than the granite dikes because the amphibolite dike cuts through the
granite (Fig. 3). Closer examination, however,
shows that the amphibolite must be older because the granite extends
through
the amphibolite (photo 7 in Fig. 3; Fig. 7). Therefore, the
geologic relationships shown in the outcrop are enigmatic. Which is older
-- the amphibolite dike or the granite dike? Moreover, if the granite
dike is younger than the amphibolite dike and is formed by magma extending
from the Woodson Mountain granodiorite (Fig. 3), a second puzzling aspect is how can a hot,
viscous, granitic liquid penetrate the quartz diorite to form the
granite dike without disrupting, breaking, or displacing the
amphibolite dike?

The textural problem that myrmekite poses for determining its
origin

Because the magmatic origin of the granite
dikes is problematic,
a metasomatic origin is a possibility. In previous studies Collins
(1988a) and Hunt et al. (1992)
suggested that myrmekite is a clue to K-metasomatic transformations. The
microcline in the granite dikes near Temecula, however, looks primary, as
if the
microcline were former orthoclase of magmatic origin. Moreover, the
wartlike myrmekite (Fig. 4), projecting into the
microcline, has the appearance of
either having been formed (a) by exsolution from the microcline or
(b) by
Ca- and Na-replacement of the microcline (Collins,
1997a, 1997c). If the hypidiomorphic
texture and mineralogy in the
myrmekite-bearing granite (Fig. 4) are compared with
the magmatic texture and mineralogy in the biotite-hornblende quartz
diorite (Fig. 5), no logical reason seems to exist
that would suggest a metasomatic origin of the granite. Nevertheless,
close
scrutiny of the textures and mineralogy of the quartz diorite and granite
provides reasons to support the field evidence of such an origin
(Collins, 1988b).

The evidence for metasomatism

Although west of the amphibolite dike (west
of photo 6, in Fig. 3), the quartz diorite is unaltered, east of the
amphibolite dike (west and north
of photo 2, in Fig.
3), it is microfractured, and its plagioclase,
hornblende, and biotite show early stages of alteration. The normally
zoned plagioclase crystals develop mottled extinction under
cross-polarized light (Fig. 8) and have lost some of
their albite twinning.

Cathodoluminescence and electron microprobe
studies of the mottled crystals show that the mottling results from
loss
of Ca along microfractures. The subtraction of Ca destroys the
zonation
and albite twinning and leaves residual Na in the altered lattice (Fig. 1 and Fig. 2 in
Collins, 1997b). Locally, in nano-sized cracks
that penetrate to the cores of the microfractured plagioclase crystals,
tiny islands of K-feldspar are found. These K-feldspar islands and
the
microfracturing (cracks) that allowed the removal of Ca and
apparent penetration by
K-replacement fluids are invisible under cross-polarized light (Fig. 8).

In the same places that broken plagioclase
crystals are modified
east of the amphibolite dike (Fig. 3), some
hornblende and biotite crystals develop a quartz sieve texture where the
interiors of microfractured crystals are replaced by quartz (Fig. 9 and Fig. 10). Quartz
would
not be expected to be in the cores of hornblende that would have
crystallized at much higher temperatures than quartz. Therefore, the
occurrence of quartz in hornblende cores must represent
Si-replacements.

&@9;In a few places, interiors of
microfractured plagioclase are
replaced by visible microcline (Fig. 11), but here
also the microfractures that allowed the removal of Ca and Na and
introduction of K cannot be observed in cross-polarized light but only in
cathodoluminescence images. If these
islands of K-feldspar were formed by magmatic processes to produce
anti-perthite, then almost all plagioclase crystals should show this
K-feldspar-plagioclase intergrowth, and that is not the case.

Mineralogical and textural changes in a 16 cm transition zone

On the west side of the amphibolite dike
where it cuts across the
granite dike (photo 7, Fig. 3), samples of the
narrow border of
remnant quartz diorite obtained between the diamond-saw cuts (Fig. 7), contain some hornblende (Fig. 9) and zoned
plagioclase (Fig. 8) similar to that in Fig. 5.
Microfractured parts of this quartz
diorite show early stages in which the normally zone plagioclase crystals
begin to lose their albite twinning and zoning. Under cross-polarized
light, they exhibit mottled extinction (Fig. 12, left
side). Electron microprobe studies show that rims of these mottled
crystals are An18-20, and cores are reduced from
An39
to An25-30 (Collins, 1988a). This bordering quartz
diorite is first in a sequence of textural and mineralogical changes that
occur westward
across an interval of 16 cm to the granite (between the saw cuts; Fig. 7). The changes are not uniform
but irregular from place to place, depending upon the local degree of
microfracturing and cataclasis.

Next in this sequence, (a) hornblende is no
longer present and
presumably replaced totally by quartz, (b) biotite is reduced in volume
and presumably partly replaced by quartz, (c) quartz increases in
volume proportional to probable losses of hornblende and biotite, and (d)
the mottled plagioclase
crystals gradually but completely lose there albite twinning and normal
zoning (Fig. 12, right side). In these places cores and rims
of some modified plagioclase crystals have about the same composition,
An18-20; others have reverse zoning with cores An15
and rims An18-20. Electron microprobe studies show that these
altered plagioclase crystals contain local concentrations of K (up to 50%
K-feldspar), but there is no optical indication of either K-feldspar or
microfractures in these altered plagioclase crystals under cross-polarized
light (Fig. 12, right side). It is apparent from
the textures and alterations that the volumes of the plagioclase crystals
remain constant, but Ca and some Al in the plagioclase interiors are being
emptied as K is introduced locally in some places.

Still farther west toward the granite, many
large plagioclase
crystals are broken into an aggregate of smaller crystals (Fig. 13, right side). All these plagioclase crystals
(large and small) are strongly altered internally as indicated by
cathodoluminescence and electron microprobe studies. The large
plagioclase
crystals (not shown in Fig. 13) and the small plagioclase crystals in the
aggregates (Fig. 13, right side) all have reversed
zoning with rims of An17-18 and cores ranging as low as
An5. Residual Na, remaining after Ca and some Al were
subtracted, makes the
cores albitic in composition (Fig. 3 in Collins, 1997b).

In thin section these altered grains have a
speckled appearance
(Fig. 12, right side, and Fig.
13) because of later low-temperature weathering (alteration) of the
rocks. Scanning electron images of these speckled grains at
magnifications of 1600x and 8000x show that these grains are full of
nano-sized holes, and that K and Ba have replaced the plagioclase along
the rims of the holes. (Fig. 4a and Fig. 4b in Collins,
1997b).

Nearer to the granite from this stage of
mineral modifications,
microcline makes its first appearance as large crystals Fig. 13, left side). On the basis that K is shown to
enter the nano-sized holes in the altered plagioclase lattice, the large
microcline crystals are interpreted to represent places where many altered
plagioclase grains (large and small) were totally replaced by K-bearing
fluids. Some altered plagioclase grains, however, did not become replaced
and either became inclusions or still remained as aggregates along
microcline borders. Some of these inclusions and altered
plagioclase crystals bordering the microcline, however, show the first
stages for their
recrystallization to become myrmekite. The first quartz vermicules in
the altered plagioclase grains (early myrmekite) initially have fuzzy
poorly-defined borders
(Fig. 13) or are represented by narrow veins with
little quartz in them (Fig. 13; lower left of
left side.

The origin of the quartz vermicules in the
myrmekite is
interpreted to result from recrystallization of the altered (speckled)
plagioclase grains that lost both Ca and some Al from their cores. That
is, the altered
plagioclase lattice has an imbalance in the amounts of residual Ca, Na,
Al, and Si that would normally all combine to produce only plagioclase
feldspar. During recrystallization, too much Si remains in the altered
lattice to utilize all of the residual Na, Ca, and Al to form only
plagioclase. The excess Si migrates to localized places to form quartz
vermicules as the remaining altered lattice recrystallizes as plagioclase,
which encloses the
vermicules. The fuzzy and narrow boundaries of the initial quartz
vermicules Fig. 13) show the early stages of the
recrystallization of the altered (speckled) plagioclase grains that are
converting to myrmekite.

The narrowness of the tiny quartz
vermicules
in
the final recrystallized myrmekite reflects the fact that the primary
plagioclase crystals had little Ca and Al in them initially (averaging
An30) in comparison to more calcic primary plagioclase species
in other terranes with greater amounts of Ca and Al and higher
An-contents. Therefore, at Temecula, where lesser amounts of Ca and Al
are lost from the altered plagioclase lattice, nut much excess Si remains
to produce quartz vermicules in myrmekite (Collins, 1988a; Hunt
et al., 1992). For examples, compare quartz vermicules in myrmekite in Fig. 19 with (a) intermediate-sized quartz
vermicules in Fig. 4 in Collins (1997a) where the primary plagioclase was
An40-50, (b) very wide vermicules in Fig. 18 and Fig. 19
in Collins (2001b where the primary plagioclase
was about An60-70, and (c) very tiny, narrower
vermicules in Fig. 2 in Collins (1997a where the primary plagioclase was about
An20.

At any rate, the transitional stages
between the
speckled
altered plagioclase grains free of quartz vermicules and the same-sized
grains with
quartz vermicules in myrmekite make it clear that the tiny myrmekite
granules bordering microcline did not result from (1) primary
crystallization from magma, (2) exsolution from primary orthoclase, or (3)
Ca- and Na-metasomatism of former primary K-feldspar (orthoclase or
microcline), but originated by recrystallization of altered primary
plagioclase
grains; see also Collins (1988a, 1997a;
Hunt et al., 1992).

Still farther west toward the granite, two
types of
quartz-bleb clusters appear in
the microcline and represents different degrees to which some of the
altered plagioclase grains (Fig. 13, right side)
were modified prior to incomplete replacement by the microcline. One
type consists of tiny quartz blebs
instead of tapering quartz vermicules in residual plagioclase (left third
and
middle third, Fig. 14 and Fig.
15). The enclosing plagioclase
fades into the microcline without a distinct border against the
microcline. The other type consists of clusters of quartz blebs in the
microcline without any evidence of being enclosed by plagioclase (left
third and right third, Fig. 14).

In the first type, the imbalance of Na, Ca,
Al,
and Si probably includes some K mixed into the lattice, as indicated by
the scanning electron images of K in rims of holes in an altered
plagioclase lattice (Fig. 4a and Fig. 4b in Collins,
1997b). The lattice in these places, however, still has excess Si to
form quartz blebs when the remainder of the lattice recrystallized as a
mixed Na-K plagioclase feldspar. K in the mixed
Na-K plagioclase feldspar allows
this feldspar (hosting the quartz blebs) to fade into the microcline as
the K-content in the altered lattice gradually increases to the amounts in
the adjacent microcline.

In the second type, the residual altered
plagioclase lattice has lost, not only Ca and Na where the K has been
introduced, but also more Al than is required to make microcline in the
same volume. If insufficient residual Al remains in the altered silicate
lattice, introduced K cannot incorporate all of the silica in this lattice
to form only microcline, and, therefore, some is left over to form
clusters
of isolated quartz blebs.

These two types of quartz-bleb clusters
were initially called
"ghost myrmekite" (Collins, 1988a), because the textures looked as
if
microcline had incompletely replaced former myrmekite, leaving it as a
ghost, but that is not the case. The "ghost" replacement
possibility is implied in some rocks, particularly when quartz
vermicules seem to continue from the margin of the myrmekite into
the microcline as strings of quartz blebs. Nevertheless, myrmekite and
these two types of
quartz-bleb clusters have their own but related origins, and the latter
two are not
"ghosts of myrmekite." What forms is a function of the degree to which
Al, Ca, and Na have been removed from the altered plagioclase lattice and
the degree to which K entered this altered lattice prior to
recrystallization, as described above.

Where the two types of quartz-bleb clusters
are found, the coexisting
myrmekite inclusions in the microcline and aggregates of myrmekite on
microcline borders have completely recrystallized (Fig. 15), and they no longer have the speckled
appearance that is observed in Fig. 12 and Fig. 13). The excess Si has completed its diffusion
from the altered lattice, where Na, Ca, Al, and Si were imbalanced, to
form
distinct quartz vermicules, and these vermicules are
slightly thicker
than in the first stages and no longer have fuzzy borders (Fig. 13).

The final mineralogical and textural
changes that produce the myrmekite-bearing granite

Finally, in the last stages of the gradual
conversion of the
altered quartz diorite to is granite (across the 16 cm of the transition), the
large altered plagioclase crystals (like that in Fig. 12, right side) have recrystallized to form
unzoned plagioclase An12-15that develops albite
twinning again
(Fig. 16). As the recrystallized plagioclase is
formed, the volume of coexisting microcline increases to greater than
50 percent. Some of these microcline crystals have aggregate myrmekite on
their borders (Fig. 16
and Fig. 17) and/or interior quartz-bleb clusters
(Fig. 18), but most are relatively clean, devoid of
inclusions or myrmekite borders. Generally, there is less than 1 vol.
% myrmekite, but included in this volume is an additional type of
myrmekite, called wartlike myrmekite. Wartlike myrmekite is
distinguished from aggregate myrmekite in that wartlike myrmekite
consists of a single quartz-plagioclase intergrowth-grain that is
attached to non quartz-bearing plagioclase while projecting into
microcline as "warts" or cauliflower-like structures. Aggregate
myrmekite is composed of many isolated grains are unattached to non
quartz-bearing plagioclase. To explain the origin of wartlike
myrmekite requires understanding how the unzoned plagioclase
An12-15 is formed.

The recrystallized plagioclase
An12-15 in both large
and small crystals in the granite results because not all (speckled)
plagioclase crystals are replaced by microcline or converted to
myrmekite or quartz-bleb clusters. Where microcline
replaces some altered plagioclase lattices, some Na that is displaced by
the K moves into other adjacent altered plagioclase lattices to
recrystallize them as a more sodic unzoned species. At Temecula, the
more-sodic
species is An12-15, but in other terranes the An values of the
recrystallized plagioclase may be higher or lower. Generally, the
An-content of the recrystallized plagioclase is about half the An-content
of the original primary plagioclase (Collins, 1988a).

Although most recrystallized sodic
plagioclase that is formed by
metasomatic processes lacks a myrmekite border against microcline, in a
few places where K happens to come into a large altered plagioclase
crystal from one side and where displaced Na comes in from the opposite
side, disproportionate amounts of Ca and Al relative to residual Na and Si
are trapped between the opposing K- and Na-replacement fronts.
Consequently, an imbalance in residual Ca, Na, Al, and Si produces an
excess of Si that forms quartz vermicules in wartlike myrmekite that
projects into the microcline while being attached to the non
quartz-bearing, recrystallized sodic plagioclase (Fig.
19).

Note that the albite twinning of the
plagioclase in the wartlike
myrmekite is optically continuous with the albite twinning in the non
quartz bearing, recrystallized plagioclase crystal outside the microcline
(Fig. 19). Moreover, the albite twinning of the
grid twinning of the microcline is also parallel to the albite twinning of
the recrystallized plagioclase (Fig. 19; see
also
Fig. 11). This parallelism strongly suggests that
the microcline is not primary orthoclase that inverted to microcline
but is secondary replacement microcline that has inherited its
lattice from a former altered plagioclase
lattice, part of which also became the wartlike myrmekite and the
non quartz-bearing plagioclase (Collins,
2000).

Because only locally are disproportionate
amounts of Ca and Al
trapped in altered plagioclase lattices during K-metasomatism, most
microcline that replaces the altered, large and small, plagioclase
grains lack aggregate and wartlike myrmekite and/or quartz-bleb
clusters. The
abundant microcline, free of myrmekite and quartz-bleb clusters, occurs
because in most places, as
K enters an altered plagioclase crystal, outlets for displaced Ca, Na,
and Al to
escape through broken boundary seals are created against all bordering
crystal types so that only microcline is formed. On the other
hand, if a primary plagioclase crystal is being altered and its crystal
boundary on one side is still sealed, for example, against adjacent biotite
or quartz crystals, then K, entering the plagioclase from one side, will
be unable to displace Ca, Na, and Al out the other side. Residual Ca and
Al will
remain,
disproportionate to the remaining Na and Si, and myrmekite is
produced. Myrmekite that is formed against biotite or quartz occurs in
some rocks, but it is not
common because plagioclase
seals against biotite and quartz are normally broken. Common places where
disproportionate amounts of Ca and Al might be trapped in altered
plagioclase
include: (1) where altered plagioclase grains were surrounded by
microcline before the Ca and Al could escape (resulting
in myrmekite and quartz-bleb cluster inclusions; Fig.
14 and Fig. 15), (2) where the Ca and
Al were
caught
between advancing K- and Na-replacement fronts from opposite sides
(producing wartlike myrmekite; Fig. 19), and
(3)
where the Ca and Al were caught between
two advancing K-replacement fronts (forming swapped myrmekite or
aggregate
myrmekite between two microcline crystals; Fig. 16, Fig. 17, and Fig. 18). Swapped
myrmekite consists of one set of myrmekite grains in the border of a
microcline crystal which projects into an adjacent microcline crystal, and
these grains alternate with another set of myrmekite grains in the other
microcline crystal which projects back into the first microcline
crystal.

Still another possibility for myrmekite
formation occurs when the
primary rock has minimal deformation, just enough to break crystal
boundary seals and fracture the margins of zoned plagioclase crystals, but
not enough to fracture the cores. In that case, an advancing K-front in
hydrous fluids, moving along the broken seals between crystals, may trap
disproportionate Ca and Al in the altered marginal lattices of the
plagioclase
and create rim myrmekite (an intergrowth of a narrow band of short
quartz vermicules in coexisting sodic plagioclase that borders
zoned
plagioclase). Rim myrmekite generally doe not project into coexisting
K-feldspar but may do so if the
metasomatic K-feldspar crystal has enough volume (e.g., Fig. 19 in Collins,
1997f). The total amount of
metasomatic K-feldspar that is formed in a rock containing rim myrmekite
is generally small (1-10 vol. %); the amount depends on the degree of
fracturing. Moreover, in some rocks where K-feldspar is scarce, rim
myrmekite may not even have a visible K-feldspar border because all that
is necessary to form rim myrmekite is the recrystallization of an altered
plagioclase lattice in which the residual Ca, Na, Al, and Si are
imbalanced such that excess Si is present to form the quartz vermicules.
Furthermore, the associated ferromagnesian silicates may show little to
no replacement by quartz. Consequently, the metasomatic alterations in
rim myrmekite-bearing rocks are nearly isochemical. Rim myrmekite is
not found in the
Temecula site
because both cores and rims of former zoned plagioclase crystals were
strongly microfractured and/or deformed.

Significance of low percentages of wartlike myrmekite

On the basis of the general openness of the
microfractured
system where K-metasomatism has occurred, the small amounts of
wartlike myrmekite
in a granitic rock (generally less than 1.0 vol. %)
are not an indication of minimal K-replacements. Instead, the
presence
of wartlike myrmekite, even in small amounts, is a signal of former
extensive microfracturing (with possible cataclasis) and vast amounts of
K-replacements that produced abundant microcline, most of which lacks
myrmekite along its borders. This relationship needs to be
recognized as well as the realization that an alteration continuum
exists between plutons with minimal K- and Si-replacements, where rim
myrmekite is formed, and plutons in which extensive K- and
Si-replacements have occurred, where abundant microcline and wartlike
myrmekite are found.

In the Temecula site, the abundance of
microcline (55-60 vol. %)
and scarcity of plagioclase (5-10 vol. %) in the granite likely reflects
the strong cataclastic granulation of the primary plagioclase
(60-75 vol. %) crystals in
the former quartz diorite. In most granitic plutons that have been
modified by K- and Si-metasomatism, however, about half of the primary
plagioclase is replaced by K-feldspar and the other half by
recrystallized sodic plagioclase. In the Temecula site, the
cataclasis probably involved
repeated episodes of deformation that kept the system open so that the
microfractured plagioclase crystals were mostly replaced by microcline,
and only a little sodic plagioclase remains.
The
great degree of K-replacement here is similar to that which produced
myrmekite-bearing pseudo-aplite dikes consisting mostly of microcline and
quartz, which occur in the Cathedral Peak granodiorite (Collins and Collins, 2002). Because most
plagioclase in the granite dike near Temecula is replaced by microcline,
it is not surprising that most primary biotite and hornblende were also
replaced by quartz. Further evidence that the granite resulted from
replacement of former quartz diorite is the local occurrence of a small
undeformed island of the former quartz diorite (4 mm wide; Fig. 20) in the same thin section that Fig. 19 was photographed. This tiny remnant quartz
diorite occurs in the midst of the
myrmekite-bearing
granite, three meters from the remnant quartz diorite that borders the
amphibolite dike (Fig. 7).

Other significant observations

A significant observation that can be made
about the sequence of
changes that are observed in Figs. 8-19 is that the final product, the
granite, is massive and exhibits no outward appearance of cataclasis or
gneissic foliation that might be expected if the former rock were
subjected to intense shearing and granulation. Most of the quartz
lacks undulatory
extinction; the microcline and recrystallized plagioclase are not
deformed; and the granite has an interlocking hypidiomorphic texture
typical of a granite crystallized from a magma. It must be recognized
that
the whole replacement and recrystallization process that produces wartlike
myrmekite, not only eliminates or greatly modifies the former precursor
minerals, but also
destroys the evidence for former strong deformation; see also Sylvester
and Christie (1968) for similar evidence of the elimination of strong
deformation in quartz by dynamic recrystallization.
At Temecula, the only bits of evidence in the granite of the former
microfracturing and cataclasis are the occurrences of myrmekite and
quartz-bleb clusters (e.g., Fig.
18 and Fig. 19). The granulated and
microfractured grains are no longer present. Therefore, the absence of
deformation in a myrmekite-bearing granite is not a criterion that
can be used to say
that K-metasomatism did not take place.

A second significant observation is that
although the
myrmekite-bearing granite has a gradational contact with remnant quartz
diorite adjacent to the amphibolite dike in the 16-cm transition zone (Fig. 7), all other granite contacts against the quartz
diorite are knife-edge sharp (photos 3, 4, and 5 in Fig. 3). Such sharp contacts between mafic and
felsic igneous rocks are commonly used as a
criterion to indicate injection of magma rather than metasomatism. But
fluids producing K- and Si-metasomatism did not penetrate the quartz
diorite where it was
undeformed, and, therefore, replacements of microfractured biotite,
hornblende, and plagioclase by quartz, microcline, and sodic plagioclase
could occur only to a sharp limit against the quartz diorite that had not
been
microfractured. At any rate,
these sharp quartz diorite-granite contacts in the Temecula site give
false evidence for
magmatism. See
Collins (1997f) and Roddick (1982).

Absence of K-feldspar megacrysts

Another significant observation is the
absence of microcline (or
orthoclase) megacrysts in the granite dikes at Temecula, even though the
K-metasomatism there was quite extensive, producing 55-60 vol. %
microcline. Why are myrmekite-bearing microcline megacrysts formed, for
example, in the Cathedral Peak granodiorite (Collins and Collins, 2002), the Twentynine Palms quartz monzonite
(Collins, 1997d), and the granodiorite near
Monterey, California (Collins, 2001a), but not
in the granite at Temecula? And, why is there lesser modal microcline in
these rocks than in the granite at Temecula? When the textures and
mineralogy in these two kinds of metasomatic granitic rocks are compared,
the rocks containing microcline megacrysts still have coexisting
normally zoned plagioclase crystals in them, but that is not the case
at Temecula. This difference is a clue for explaining why megacrysts were
formed in the above three terranes but not at Temecula.

Microfracturing of plagioclase in the
primary rocks in which the
microcline megacrysts grew, began selectively in a few scattered zoned
plagioclase crystals but not in all. As these microfractured plagioclase
crystals were replaced in their interiors by microcline and grew in size,
subsequent microfracturing occurred in zoned plagioclase crystals only
adjacent to the border of the growing microcline crystal but not beyond
the border into the other groundmass zoned plagioclase crystals.
Consequently, zoned plagioclase crystals still remained in the rocks
because K-bearing fluids could not enter unbroken plagioclase to cause
replacements. In these rocks where only a few isolated zoned plagioclase
crystals were microfractured, fewer sites for nucleations and
K-replacements occurred, and the introduced K was concentrated on these
fewer sites to make megacrysts. The lesser amounts of microfracturing in
these rocks also limited the opportunities for K-metasomatism, and,
therefore, the modal volume of the microcline in these rocks is less than
in the granite at Temecula.

In contrast, in the primary quartz diorite
at Temecula, the
cataclasis was much stronger, and all zoned plagioclase crystals were
microfractured to their cores, perhaps nearly at the same time. In that
case, where all plagioclase crystals were microfractured, introduced K had
multiple, simultaneous places for K-replacements to occur. All these
places competed for the K, and none formed microcline that was able to
grow large enough to become megacrysts. Furthermore, the greater
microfracturing and cataclasis enabled more opportunities for
K-replacements and produced greater volumes of microcline.

Conclusions

Knowledge of the sequential
changes in Figs. 8-19 that led to the formation of myrmekite-bearing
granite should enable the geologist investigating other terranes to
recognize K-metasomatism on a plutonic scale in granitic plutons, which
heretofore might have been considered to have only a magmatic history.
Criteria that can be used to help recognize this possible K-metasomatism
include: (1) microcline in cores of some plagioclase crystals (Fig. 11, left side, and Fig.
17),
(2) isolated irregular islands of microcline in plagioclase
(Fig. 11, right side; Fig.
6 in Collins, 1997b), (3) the
parallel
alignment of albite
twinning in the grid-twinning of the microcline with albite twinning in
plagioclase outside the microcline (Fig. 11, left
side,
and Fig. 19), (4) the occurrence of
plagioclase
inclusions in microcline which are in parallel optical continuity with a
plagioclase crystal outside the microcline (e.g., Fig. 10 in Collins, 1997d), and (5) the presence of both
quartz-bleb
clusters and wartlike myrmekite (Fig.
14). See Fig. 5 and Fig. 6 in Collins 1997e for an examples of coarse quartz blebs in
clusters ("ghost myrmekite") and Fig. 18
and Fig. 19 in Collins ((2001b) for examples of myrmekite with very coarse
quartz vermicules that are transitional to coarse quartz-bleb
clusters.

Acknowledgments

Thanks are given to Forrest Hopson and Karl
Ramseyer for the
cathodoluminescence images and to David Liggett for help in
putting the article on the Internet.